Proteases have long been employed in industry for a wide variety of products, including detergents (e.g., laundry detergents), fiber-modifying agents, leather treatment agents, cosmetics, bath agents, food-modifying agents, and drugs. Among these, proteases for detergents are industrially produced in the greatest amounts. Examples of such proteases known heretofore include Alcalase®, Savinase® (Novozymes), Maxacal® (Genencor), Blap® (Henkel), and KAP (Kao Corporation).
Protease is incorporated into a laundry detergent for providing the detergent with the ability to degrade dirt mainly composed of protein and deposited on clothing into low-molecular-weight products, to thereby promote solubilization of the thus-degraded products with a surfactant. However, in actuality, such deposited dirt is complex dirt containing, in addition to proteins, a plurality of organic and inorganic components such as sebum-derived lipid and solid particles. Therefore, there is a continuous demand for a detergent exhibiting excellent detergency to such complex dirt.
In view of the foregoing, the present inventors previously discovered several alkaline proteases having a molecular weight of about 43,000, which maintain sufficient casein-degrading activity even in the presence of a fatty acid of high concentration and which exhibit excellent detergency to complex dirt containing proteins and sebum; and previously filed a patent application on the alkaline proteases (Patent Document 1). These alkaline proteases differ from conventionally known subtilisin, a serine protease derived from bacteria belonging to the genus Bacillus, in terms of molecular weight, primary structure, and enzymological characteristics, and having a very strong resistance to oxidizer. These alkaline proteases are suggested to be classified into a new subtilisin subfamily (Non-Patent Document 1).
Meanwhile, detergents can be categorized, by form thereof, into powder detergents and liquid detergents. Advantageously, liquid detergents have solubility higher than that of powder detergents, and neat liquid thereof can be directly applied to dirt. Although liquid detergents have such merits while powder detergents do not possess, liquid detergents are widely known to encounter technical difficulty in stable incorporation of an enzyme such as protease, while powder detergents do not encounter. Generally, since liquid detergents are stored at ambient temperature, the enzyme (protein) is readily denatured. In addition, liquid detergents contain a surfactant, fatty acid, solvent, etc., and the pH thereof falls within a weak alkaline range. Such conditions are very severe conditions for the enzyme. Furthermore, the protease, which is a proteolytic enzyme, undergoes problematic self-digestion, further reducing storage stability of the enzyme in liquid detergents.
In order to solve the aforementioned technical problems, there have been widely known addition of an enzyme-stabilizing agent such as calcium ion, borax, boric acid, a boron compound, a carboxylic acid (e.g., formic acid), or a polyol. Some studies have been carried out to cope with the problem of self-digestion based on inhibition of protease activity. Specifically, there have been reported methods for stabilizing protease through reversible inhibition of protease activity by use of 4-substituted phenylboronic acid (Patent Document 2) or a certain peptide-aldehyde and a boron composition (Patent Document 3). Also reported is that dextran-modified protease enhances stability of protease in aqueous solution containing a surfactant (Non-Patent Document 2).
However, the protease-stabilizing effect due to addition of an enzyme-stabilizing agent (e.g., calcium ion or boric acid) is insufficient, and the inhibitory effect varies depending on the type of protease. Furthermore, use of such agents increases production cost. Thus, these countermeasures are not thought to be best solutions for the problems involved in liquid detergents. Chemical modification of the enzyme also has problems in terms of production cost.
Generally, a surfactant, an alkaline agent, an anti-redeposition agent, solvent, perfume, a fluorescent dye, etc, are added to liquid detergents. Among these additives, a surfactant most severely impairs the stability of enzymes. Typically, an anionic surfactant and a nonionic surfactant are used in combination. Although a nonionic surfactant does not greatly damage enzymes, an anionic surfactant is thought to greatly damage enzymes, since the anionic surfactant enters the enzyme via its hydrophobic moiety and breaks hydrophobic interaction of the enzyme as well as traps divalent metal ions (e.g., calcium ions) which stabilize the enzyme (Non-Patent Document 3). Thus, enhancement of resistance of the enzyme to anionic surfactants is a very important factor for enhancing the stability of the enzyme in liquid detergents.
In an alkaline protease derived from KP43 [Bacillus sp. KSM-KP43 (FERN BP-6532)], the specific activity to the activity of the parent alkaline protease is known to be enhanced through substitution of the amino acid residue at the position 15 of the amino acid sequence with a histidine residue; substitution of the amino acid residue at the position 16 of the amino acid sequence with a threonine or glutamine residue (Patent Document 4); substitution of the amino acid residue at the position 65 of the amino acid sequence with a proline residue (Patent Document 5); or substitution of the amino acid residue at the position 66 of the amino acid sequence with an aspartic acid residue (Patent Document 6). However, there has never been known an alkaline protease variant which enhances the stability of an alkaline protease derived from KP43 in liquid detergents without reducing the specific activity.    Patent Document 1: WO 99/18218 pamphlet    Patent Document 2: JP-A-H11-507680    Patent Document 3: JP-A-2000-506933    Patent Document 4: JP-A-2004-305176    Patent Document 5: JP-A-2004-000122    Patent Document 6: JP-A-2002-218989    Non-Patent Document 1: Saeki et al., Biochem. Biophys. Res. Commun., 279, 313-319, 2000    Non-Patent Document 2: Cosmetics & Toiletries magazine, 111, p. 79-88, 1996    Non-Patent Document 3: Detergent Enzyme: A Challenge! In Handbook of Detergents part A, New York, p. 639-690, 1999